My name is Shane, and I bought my son, Mason, who is 2, an ActionWheels 6v Batmobile for Christmas 2016. Here is a pic from WalMart: I didn't recognize this batmobile model, until I started playing Arkham Asylum: There is a HotWheels toy: And TheBrickBuilder14 created a really cool custom Lego model:

With only 6 volts, it's speed was very slow, at 2.5mph, measured using a speedometer app on my phone. I've upgraded it with a 2nd 6v motor/gearbox, wired backwards, in parallel, purchased from Ballard Pacific.

Running it with a single 6v battery, it ran at the same speed, it seemed to drive better in the grass, but had a very shortened run time.

Adding a 2nd 6v battery in series, doubled the speed, but still had a shortened run time.

My plan is to add 2 x 12v batteries, in parallel, for double the speed, and normal run time.

I'll post pics of the modifications I had to do to the axle, to accommodate the 2nd gearbox/motor, and the custom wire harness I built.